Center opening elevator doors systems consist of a pair of car doors and a pair of hoistway doors. Each pair of doors opens and closes about a central point in an elevator doorway.
A door operating unit, disposed atop an elevator car, is attached to each car door via a four bar linkage. Each car door, in turn, is coupled by a pair of linked rollers to a vane attaching to the hoistway doors.
As each car door opens or closes, driven by the door operating unit via the four bar linkage, a first of the linked rollers engages the vane. Typically, contact with the vane by the first roller urges the second roller into contact with the vane as the car door moves thereby locking the vane between the two rollers. When both rollers engage the vane, the hoistway door starts to move with the car door. Because the car door starts to move before the hoistway doors, the car door leading edge is offset from the hoistway door leading edge during portions of the travel of the doors. As a result, the door operating unit must provide force to accelerate the car door and then provide an additional force to accelerate the attached hoistway door. The additional force may result in noisy door operation unless the motion of both doors is accurately controlled both in speed and distance.